The invention relates to a master cylinder for a vehicle hydraulic brake system, comprising a housing with a bore formed therein, which is open at one end and in which a first pressure piston and a second pressure piston are guided sealingly and displaceably along an axis of the master cylinder. Acting counter to an actuating direction upon the first pressure piston is a restrained first restoring spring and upon the second pressure piston a second restoring spring. The term “restrained” means that the first restoring spring may expand, not freely, but only to a specific extent that is defined by a restraint arrangement, which restrains the restoring spring.
Master cylinders of the described type have been known for some time. In the bore, the two pressure pistons delimit a first and a second pressure chamber, which in the operable state of the master cylinder are full of hydraulic fluid. Usually, the first pressure chamber is fluidically connected to a first brake circuit and the second pressure chamber to a second brake circuit of a vehicle brake system. In each brake circuit a plurality of wheel brakes are disposed. During a braking operation, the first pressure piston is displaced into the bore of the master cylinder housing in order to generate hydraulic pressure in the first pressure chamber. A hydraulic pressure arising in the first pressure chamber also displaces the second pressure piston further into the bore so that hydraulic pressure builds up likewise in the second pressure chamber. The hydraulic pressure generated in the two pressure chambers is supplied to the appropriate wheel brakes via the brake circuits connected to the pressure chambers. On completion of a braking operation, the force exerted by the two restoring springs effects a return of the pressure pistons to a normal and/or inoperative position. The exact function of such a master cylinder is well known to experts in this field and therefore requires no further explanation here.
The already mentioned restraint arrangement of the first restoring spring is used to adjust the position of both pressure pistons relative to one another. A precise adjustment of the position of the two pressure pistons relative to one another in the inoperative position is important so that during a braking operation the pressure build-up occurs simultaneously in both pressure chambers. The restraint arrangement limits the expansion of the first restoring spring and hence fixes the distance that exists in the inoperative position between the first pressure piston and the second pressure piston.
When the pressure pistons are in the said inoperative position, the restoring springs are not fully relaxed. This leads, when the master cylinder is not installed in the vehicle, i.e. when for example it is being transported to the vehicle manufacturer or disassembled for servicing purposes, to the unrestrained second restoring spring displacing the second pressure piston counter to the actuating direction in the bore. This movement is transmitted via the first restoring spring to the first pressure piston, which therefore in the worst-case scenario may drop entirely out of the bore of the master cylinder housing. Shaking and vibration during transportation additionally promote this undesirable occurrence.
For solving this problem, the use of a transport lock is known in principle. Conventional transport locks however have the drawback of being more or less elaborate and therefore time-consuming to remove before installation of the master cylinder into a vehicle may occur. In the case of disassembly of a master cylinder for servicing purposes, moreover, there is no longer a transport lock available for use.